Conventionally, an epoxy resin composition for casting has been used to insulate and protect coils or motors incorporated in electrical or electronic equipment, and electric conductors of solid insulated switching device.
Generally, the epoxy resin composition for casting contains an epoxy resin, a curing agent, and silica. In the case where the epoxy resin composition contains silica, the epoxy resin composition has a lower thermal expansion coefficient, and the difference between the thermal expansion coefficient of the epoxy resin composition and the thermal expansion coefficient of a metal part to be insulated and protected becomes small.
As a result, stress generated between the metal part and the epoxy resin composition also becomes small, so that crack hardly generates.
Recently, the thermal shock resistance in addition to the heat resistance has been required for the epoxy resin composition for casting due to high voltage loading, preparation of thinning material such as miniaturization and weight reduction in casted products. However, the thermal shock resistance always has no relation to the impact resistance.
Therefore, it is difficult to improve the thermal shock resistance, and various studies have been carried out. For example, Patent Document 1 discloses that a composition contains an epoxy resin, silica, and a polyether polyol having molecular weight of 1000 or less, and the composition has effects on internal stress relaxation. However, the thermal shock resistance cannot be sufficiently improved in the cured product formed from the composition of Patent Document 1.
Patent Document 2 discloses that a composition contains a liquid type butadiene and acrylonitrile rubber (CTBN) having a carboxylic acid at terminal together with silica in a large amount and a polypropylene glycol having molecular weight of 1000. While the composition of Patent Document 2 has the improved thermal shock resistance to some degree, the thermal shock resistance is still insufficient and the heat resistance is greatly decreased.
Patent Document 3 discloses that a composition contains an epoxy resin, a core-shell polymer, and a polyol. However, in Patent Document 3, the polyol is described as in the same manner as a curing agent, and there are no descriptions of the purpose to be used and effects thereof. Actually, even if the composition is prepared based on Patent Document 3, it is difficult to prepare a cured product having excellent thermal shock resistance.